As a result of expected stricter legislation concerning the levels of nitrogen oxides (NOx) in exhaust gases from diesel engines, extensive development is currently being carried out worldwide to produce a system for the cleaning of exhaust gases and reduction of NOx levels in such diesel engines. Much of the development effort is focused on exhaust gas cleaning technology in turbocharged diesel engines for use in heavy-duty (i.e., on road) applications (e.g., heavy duty truck engines).
Several solutions have been suggested, including catalysis processes which include several subprocesses (for example, injection of water, addition of urea) that in addition to being expensive also involve disadvantages in the form of complex and space-demanding components. The EGR (exhaust gas recirculation) concept has attracted interest since it not only has advantages from the point of view of expense but also is expected to offer safe functioning and simple and compact construction.
During the turbocharging of heavy diesel engines that takes place when in operation, the pressure of the exhaust gases in most cases is less than the inlet pressure, and exhaust gases can therefore not be recirculated without measures being taken for achieving a sufficient supply of exhaust gases. Such measures may take the form of, for example, venturi solutions, exhaust throttles or inlet throttles. However, these solutions have up until now been associated with disadvantages in the form of, for example, reduced engine power through high pressure losses, together with increased fuel consumption and smoke development.
By placing a venturi in the inlet flow, an advantageous difference in pressure between the exhaust channel side and the inlet channel side is achieved locally in the venturi, and exhaust gases, which are removed upstream of the turbo, can be fed into the inlet pipe of the engine. A reduced NOx level is obtained as a result of the resulting lower combustion temperature.
U.S. Pat. No. 5,333,456 (Bollinger) discloses a flow valve in the form of a coil that is placed upstream in the EGR supply flow. This control valve is not configured in a way which would allow it to be used in the inlet channels of turbocharged engines.
U.S. Pat. No. 5,611,203 (Henderson, et al.) discloses a flow regulator with venturi function which is disposed in the inlet channel next to the EGR supply flow. The opening for supply of exhaust gases is not located where the throttling of fresh air is greatest, which would involve a more severe throttling than necessary, while the total pressure losses, which arise from, for example, the neighboring actuator, become significant.
The publication SAE 2000 World Congress, SAE Technical Paper Series 2000-01-0225 discloses a variable venturi with axial EGR supply. The design does not display a proper venturi shape since the fresh air is exposed to a momentary increase in area at the end of the injector pipe, and therefore pressure losses can be significant. The component must be equipped with an elbow, with its associated pressure losses, as a result of the axial supply. Moreover, the dimensions of the component are unnecessarily bulky, which is also disadvantageous. The arrangement is primarily intended for measurement purposes and has no interest with respect to normal operating conditions.
The afore-mentioned U.S. patent application Ser. No. 10/363,350, filed Jul. 7, 2003, which is hereby incorporated by reference in its entirety, obviates these problems associated with the above-described prior art by providing an EGR system which includes a streamlined body arranged to be displaced in the longitudinal direction of a line at the inlet thereof. The body allows for achievement of a variable venturi effect and in this way a variable suction effect and mixture of the mixed flow. The system also includes an actuator for displacing the body forwards and backwards in the line.
While the EGR system disclosed in U.S. patent application Ser. No. 10/363,350 provides excellent results in connection with diesel engines, the application does not address use of the device disclosed therein in connection with gasoline engines, which engines exhibit performance characteristics and associated problems not encountered in diesel engines. One of the distinctions between gasoline and diesel engines is that gasoline engines control power output with a throttle which generally decreases engine efficiency, while diesel engines do not have a throttle. Another distinction is that gasoline engines have knock problems when one attempts to increase efficiency (i.e., by raising compression ratio, advancing timing, etc.); diesel engines do have such knock problems. A further distinction is that gasoline engines rely on three-way catalytic converters (three-way referring to the treating of three emissions: nitrogen oxides, hydrocarbons and carbon monoxide) for emission reduction, which catalysts do not work during cold start or during rich running conditions, while diesel engines do not use such three-way catalysts. Yet another distinction is that gasoline engines produce emissions of hydrocarbons (HC), carbon monoxide (CO) and NOx, while diesel engines do not emit any significant amount of HC or CO (but much more NOx and particulates).
While attempts have been made to employ EGR systems in connection with gasoline engines, such attempts have generally met with limited success and have focused only on reducing NOx (which as described above is more of a concern with respect to diesel engines than with respect to modern gasoline engines), rather than addressing problems facing gasoline engines. Such deficient attempts are disclosed in U.S. Pat. No. 4,174,027 (Nakazumi) and U.S. Pat. No. 4,224,912 (Tanaka). However, since the use of catalytic converters in connection with gasoline engines became popular, few if any attempts have been made to employ EGR systems in connection with gasoline engines (since conventional wisdom has always been that EGR systems are only useful for reducing NOx, and since the catalytic converters dealt with NOx reduction so well that additional NOx reducing measures were not necessary).
The publication SAE Technical Paper Series 982476, titled “Knock Suppression in a Turbocharged SI Engine by Using Cooled EGR”, discloses that due to the high power density of turbo-charged engines, knocking combustion and high exhaust gas temperatures constitutes a problem at high loads. The reference also teaches that in order to reduce the gas temperature and to suppress knock, excessive amounts of fuel have traditionally been used. The references teaches that knock suppression and cooler exhaust gas temperatures can be achieved by using cooled EGR systems in connection with turbo-charged gasoline engines, rather than the traditional employment of excessive amounts of fuel. The publications SAE Technical Paper Series 1999-01-3505, titled “Replacing Fuel Enrichment in a Turbo Charged SI Engine: Lean Burn or Cooled EGR”, and SAE Technical Paper Series 2003-01-0629, titled “Dilution Interest on Turbocharged SI Engine Combustion” disclose similar subject matter.
The publication SAE Technical Paper Series 970505, titled “Comparing Lean Burn and EGR”, compares the technique of lean burn with that of EGR, and determines that fuel consumption is almost as good employing EGR as it is with lean combustion. The publication SAE Technical Paper Series 961953, titled “A Strategy to Improve the Efficiency of Stoichiometric Spark Ignition Engines”, discloses that EGR as a knock reducing mean makes advanced ignition and higher compression possible. This increases engine efficiency across the complete engine load range. Furthermore, pumping losses are reduced on part load because of higher manifold pressure. The publication SAE Technical Paper Series 950684, titled “Improving NOx and Fuel Economy doe Mixture Injected SI Engine with EGR”, discloses a gasoline engine employing an EGR system in which a mechanically driven mixture injection valve and a small mixture chamber are provided into the cylinder head in order to create a stratified mixture of up to 50% EGR along with combustion air.
While the six publications described in the two preceding paragraphs do recognize some potential benefits of using EGR in connection with gasoline engines, none of the references discloses an efficient and/or effective way for introducing EGR and/or mixing EGR with air before the mixture is combusted. Moreover, the references recognize only some of the benefits achievable with employing an EGR system with a gasoline engine.
The applicants of U.S. patent application Ser. No. 10/363,350, now U.S. Pat. No. 7,036,529, have discovered that the EGR system disclosed therein provides new and unexpected results when employed in connection with gasoline engines, which results have not been attainable with any EGR system heretofore used in connection with gasoline engines.